<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<span id="s1">100</span>
</body>
<script>
    var i=100;
    function overtime(){
        document.write("活动结束");
    }
    setTimeout(overtime,100000);
    function changetime() {
        s1.innerHTML=--i;

    }
    setInterval(changetime,1000);

</script>
</html>